In the optical communications space, various techniques are used to synthesize an optical communications signal for transmission. FIG. 1A illustrates a popular technique in which a laser 2 is coupled to an optical modulator 4. The laser 2 generates a narrow-band continuous wave (CW) optical carrier signal 6 having a desired wavelength. The optical modulator 4 operates to modulate the amplitude and/or phase the carrier signal 6 to generate an optical channel signal 8, based on a drive signal 10 that encodes data to be transmitted. The optical channel signal 8 is transmitted through an optical fibre link 12 to a receiver 14. Typically, the optical fibre link 12 will include multiple optical fibre spans cascaded in series with various optical equipment including, for example, WDM Mux/Demuxers, optical amplifiers, Optical Add-Drop Multiplexers (OADMs) etc.
Typically, the drive signal 10 is generated by a driver circuit 16, which normally includes a Digital to Analog Converter (DAC) 18, a Low-Pass Filter (LPF) 20 and a Low Noise Amplifier (LNA) 22.
Referring to FIGS. 1B-1D, the DAC 18 operates to convert an input digital signal x(n) into a corresponding analog signal having a spectrum of the type illustrated in FIG. 1B. As is well known in the art, the spectrum comprises a baseband signal 24 between 0 and ±Fs/2 Hz (where Fs is the sample rate of the DAC) and harmonic bands 26 at frequencies beyond ±Fs/2. As may be seen in FIG. 1C, these harmonic bands 26 are suppressed by the Low-Pass Filter 20, having a filter characteristic 28 with a 3 dB cut-off at or near Fs/2, so that the amplified drive signal 10 output from the LNA 22 is dominated by the baseband signal 24, as may be seen in FIG. 1D. Modulating the CW carrier 6 using the drive signal 10 results in a modulated channel signal 8 having a spectrum closely similar to that shown in FIG. 1D. This channel signal 8 can be multiplexed with other channel signals in a manner well known in the art to produce a Wavelength Division Multiplexed (WDM) signal (FIG. 1E) for transport through the optical fiber link 12.
As may be seen in FIG. 1D, the spectrum of the drive signal (and thus also of the optical channel signal 8) contains out-of-band noise 30 lying at frequencies beyond ±Fs/2. This out-of-band noise 30 is the residual portion of the harmonic bands 26 that was incompletely suppressed by the LPF 20. The presence of this noise is primarily due to the fact that the filter characteristic 28 of the LPF has a finite roll-off beyond the 3 dB cut-off frequency, as may be seen in FIG. 1C.
As is well known in the art, the out-of-band noise 30 can interfere with adjacent channels of a WDM signal. Typically, this problem is addressed by designing the optical communications system to provide a guard-band 32 between adjacent channels, as may be seen in FIG. 1E. The width of the guard band 32 can be selected so that most of the energy represented by the out-of-band noise 30 lies within a guard band 32 rather than an adjacent channel. A limitation of this solution is that each guard band 32 represents un-used spectral capacity of the optical communications system. It would be desirable to use this spectral capacity for carrying subscriber data.
One known approach to addressing this limitation is to adjust the filter characteristic 28 of the LPF 20 so that the 3 dB cut-off lies below Fs/2. This has the effect of more strongly suppressing the harmonic bands 26, and so reduces the out-of-band noise 30, but at a cost of also suppressing frequency components of the baseband signal 24 near ±Fs/2. In some cases, this can create a difficulty in that clock and carrier recovery circuits in the receiver 14 may need the frequency content of the baseband signal near ±Fs/2 in order to reliably compensate phase and frequency jitter in the received optical signal. This imposes a limitation on the extent to which a real filter 20 can be used to suppress the harmonic bands 26 and therefore limit out-of-band noise 30 in the optical signal 8. Consequently, conventional optical communications networks operate with a compromise solution in which spectral capacity of the network is sacrificed in order to maintain accurate clock and carrier recovery.
Techniques that overcome at least some of the above limitations would be highly desirable.